The present invention relates to a magnetoresistive sensor with a layer made of a sensor material that possesses a perovskite-like crystal structure and exhibits an increased magnetoresistive effect.
The general structure and operation of magnetoresistive sensors with thin films made of ferromagnetic transition metals are explained further in, for example, the book "Sensors", Vol. 5, 1989, pp. 341-380. The layers of the sensors disclosed in that reference are largely free of magnetostriction and consist, for example, of a special NiFe alloy (Permalloy) or a special NiCo alloy, and however exhibit only a relatively small magnetoresistive effect M.sub.r of approximately 2 to 3%. In this context, M.sub.r =[R(0)-R(B)]/R(0), where R(B) is the electrical resistance in a magnetic field with induction B, and R(0) is the resistance in the absence of a magnetic field. The magnetoresistive effect is also sometimes defined as follows: EQU M.sub.r '=[R(0)-R(B)]/R(B); i.e. M.sub.r =M.sub.r '/(1+M.sub.r')
There is interest in increasing this magnetoresistive effect in order to produce sensors with an improved signal-to-noise ratio and expand the range of applications for such sensors. An increase in magnetoresistive effect has been detected in several multilayer systems such as Co/Cu, Co/Ru, Co/Cr, and Fe/Cr (cf. for example "Applied Physics Letters", Vol. 58, No. 23, Jun. 10, 1991, pp. 2710-2712; or "Physical Review Letters", Vol. 64, No. 19, May 7, 1990, pp. 2304-2307). These are based on the fact that a nonmagnetic intermediate layer between layers of ferromagnetic material can cause exchange coupling (exchange interaction). This coupling depends on the thickness of the intermediate layer, and requires thicknesses in the nanometer range. Exchange coupling is responsible for the magnetic characteristics ("ferromagnetic" or "antiferromagnetic") of the multilayer system.
Multilayer systems with different polarization directions for the superimposed individual ferromagnetic layers that are separated by nonmagnetic layers can accordingly exhibit an increased magnetoresistive effect M.sub.r. This effect, which can be up to 40% for sandwiched Cu-Co thin-film structures at room temperature (cf. the earlier citation from "Applied Physics Letters", Vol. 58), is therefore referred to as the "giant magnetoresistive" (GMR) effect (cf. "Physical Review Letters", Vol. 61, No. 21, Nov. 21, 1988, pp. 2472-2475).
However, the limitation to multilayer systems and strong dependence of the effect on the very low thickness (nanometer range) of the magnetic and nonmagnetic layers, makes heavy demands on the layer preparation technique and restricts the range of application to corresponding thin-film structures.
Investigations which indicate that a magnetoresistive effect can also occur, for example, in granular material systems (cf. "Physical Review Letters", Vol. 68, No. 25, 1992, pp. 3745-3752) are also known. According to these investigations, which concern the Cu-Co material system, CuCo alloy layers are produced by simultaneous sputtering of the elements, and nanocrystalline (magnetic) Co precipitates in a (nonmagnetic) Cu matrix are produced by subsequent heat treatment. According to the aforesaid citation from "Physical Review Letters", Vol. 68, the magnetoresistive effect that can be measured in these thin films is approximately 7% at room temperature.
Substantially greater magnetoresistive effects have also been observed in other ferromagnetic material systems. This applies to single crystals of the material system Eu.sub.1-x Gd.sub.x Se (cf. "Journal of Applied Physics," Vol. 38, No. 3, Mar. 1, 1967, pp. 959-964). A corresponding effect is also evident in Nd.sub.0.5 Pb.sub.0.5 MnO.sub.3 crystals; these crystals have a perovskite-like structure (cf. "Physics B," Vol. 155, 1989, pp. 362-365). However, the change in electrical resistance as a function of magnetic induction observed in these material systems is confined to low temperatures that are well below room temperature.